1. Technical Field
The present disclosure relates to a control circuit. More particularly, the present disclosure relates to a control circuit for controlling a constant output current of a converter.
2. Description of Related Art
A primary side regulator (PSR) can control an output current of a power converter. Specifically, the power converter includes a transformer. The conventional control approach for the primary side regulator is to control a conduction period of a power switch connected to a primary winding of the transformer so as to control an inductor current of the power converter. Accordingly, a constant output current of the power converter is achieved. Since the primary side regulator does not control the converter according to a feedback current from a secondary winding of the transformer, the circuitry of the primary side regulator is simple and the routing cost of the primary side regulator is low. However, the conventional primary side regulator has a defect. When the power switch is cut off, an error in the inductor current is occurred because of the effect of the propagation delay of the circuitry in the power converter, which results in an inaccurate cut-off point of the power switch and a variation in the output voltage and current. Accordingly, line regulation of the power converter is degraded.
As shown in FIG. 1, in a waveform 110 of an inductor current, the peak value of the inductor current is viewed as Ipeak1. However, because of the effect of the propagation delay Td of the circuitry, the actual peak value of the wave 110 of the inductor current should be Ipeak2. Furthermore, when the input voltage increases or a tolerance of the inductance is occurred, waveform of the inductor current is changed to a waveform 120, and thus the actual peak value of the inductor current is changed from Ipeak1 to Ipeak3. In other words, an error amount of the inductor current is increased along with the increased output voltage. In each of the waveform 110 and the waveform 120, an error caused by the propagation delay Td results in variation of the output current and degradation of the line regulation. The conventional approach is to add a saw-limit compensation circuit for correcting an error of the output current. However, in this way, the tolerance of the output current of the power converter still fails to be corrected effectively and precisely.